Tommotian
fauna

The
Tommotian Fauna - the
small shelly animals

The
Precambrian-Cambrian transition (just prior to the Cambrian
Explosion) is characterized by the appearance of small
shelly fossils that are called the Tommotian fauna after
the area of Siberia
where they were first discovered. These small shelly animals
were millimeters in size and represent the first appearance
of diverse
skeletal material in the fossil record, some 10 million years
before the first trilobites appear
in the fossil
record.
This
fauna that existed some 530 million years ago is of essential
evolutionary importance in that these are the oldest known
metazoans (animals)
that had mineralized (skeletal) hard parts, and thus are the
probable ascendants to the many phyla of the Cambrian Explosion.
They appear in the late Ediacaran
Period, close to 550 million
years ago, and some 20 million years before the soft tissue
Ediacaran Biota. Their fossil remains are found throughout
the world, so their radiation must have been extensive.
They persisted into the early Cambrian, and have been discovered
in trilobite
fossil
beds.

Many
of the 525 million year old Chengjiang
Biota, for example, including arthropods, brachiopods,
archaeocyathids, molluscs, echinoderms, and some enigmatic
animals, possessed
mineralized
body parts. Though well studied, the ancestral affinities of
many of these Tommotion metazoans
remain unknown. Prior to the Tommotian fauna, the Ediacaran metazoans
were all soft-bodied. Some of the Tommotion small shells appear
to be worm teeth; such toothed worms (like the annelids) might
have been effective predators of the soft-bodied Ediacaran life
forms.
The small
shelly animals appear to be related to diverse organisms such
as primitive annelids, lobopodians, sponges,
mollusks, and other forms that played a role in the early Cambrian
and did not persist.
The Tommotion
age is believed by some scientists to be separated from the rest
of the early Cambrian by the End-Botomian extinction event (from
about 524 to 517 million years ago in the early Cambrian) that
eliminated many Tommotian forms, which were then followed by
more familiar
early taxa of the Cambrian
Explosion.